Carbon nanotubes are cylindrical carbon molecules or structures with novel properties that make them potentially useful in a wide variety of applications. They exhibit extraordinary strength and unique electrical properties and are good conductors of heat.
Carbon nanotubes can occur in single-walled and multi-walled forms. The structure of a single-walled nanotube can be conceptualized by wrapping a 1 atom-thick layer of graphite/graphene (typically of a plurality of hexagonal unit cells) into a seamless cylinder. The way the conceptualized graphene sheet is wrapped can determine different resultant properties of the carbon nanotube, for example electrical properties such as being one of conductive, insulative, or semiconductive. Multi-walled nanotubes can be conceptualized as multiple layers of graphite/graphene rolled in on themselves to form a tube shape. The interesting combination of electronic and mechanical properties of carbon nanotubes has lead to wide ranging proposals for their potential use in future electronics and computing, field emitter devices, sensors, electrodes, high strength composites, and storage structures of hydrogen, lithium and other metals.
Many techniques have been developed to produce carbon nanotubes in sizeable quantities, including arc discharge, laser oblation, high pressure carbon monoxide, and chemical vapor deposition which may or may not be plasma enhanced. Chemical vapor deposition of carbon nanotubes presently typically reacts a carbon-containing gas (i.e., acetylene, ethylene, ethanol, etc.) with a metal catalyst particle (usually cobalt, nickel, iron, molybdenum, or a combination of these such as cobalt/iron or cobalt/molybdenum) at some suitable elevated temperatures, typically above 600° C. It is presently believed that the catalyst on the substrate for carbon nanotube growth needs to be in the form of particles instead of smooth/continuous films, although the invention as disclosed and utilized herein does not preclude use of later-developed such films or non-particle-like growth mechanisms.
Further, present generation carbon nanotube growth correlates catalyst particle size with the diameter of the resulting nanotubes. Carbon nanotubes might be one nanometer or less in diameter, and can be grown to several microns in length. Further and accordingly, a challenge exists in positioning of catalyst particles for growth of carbon nanotubes in desired locations on substrates.
Carbon nanotube growth from catalyst particles typically occurs by one of base or tip growth mechanisms. In a base growth mechanism, the particle typically remains stationary, and a carbon nanotube grows outwardly therefrom. In a tip growth mechanism, the catalyst particle typically moves away from the base substrate such that the carbon nanotube grows from the substrate inwardly of the catalyst particle with the particle being received at the outermost end of the growing nanotube. For example, a hydrocarbon such as methane adsorbed onto the catalytic particle surface can release carbon upon decomposition which dissolves and diffuses into the particle. When a supersaturated state is reached, carbon precipitates in a crystalline tubular form. At this juncture, two different scenarios are possible. If the particle adherence to the surface is strong, then carbon precipitates from the top surface of the particle and the filament continues to grow generally perpendicular to the substrate with the particle anchored to the substrate. This is called the base growth model. In cases where the particle attachment to the surface is weak, then carbon precipitation occurs at the bottom surface of the particle and the filament lifts the particle as it grows generally perpendicular to the substrate. In this case, the top end of the filament is decorated with the catalyst particle, and is referred to as the tip growth model.
While the invention was motivated in addressing the above identified issues, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.